For example, a company called Universal Containers might want to use the subdomain universalecontainers. The company’s login URL would be https://universalcontainers.my.salesforce.com/. You can use up to 40 characters.

Salesforce is automatically enabled as an identity provider when a domain is created. After your domain is deployed, you can add or change identity providers and increase security for your organization by customizing your domain’s login policy.